An internal combustion engine using gasoline as a fuel which involves resource problems is able to use chemically producible methyl alcohol (hereinafter simply referred to as "alcohol"), as is, as a fuel by merely adjusting the combustion conditions such as ignition timing and air fuel ratio, without mechanical modification.
Use of alcohol, which has a possibility to have lower market prices than gasoline in other than oil producing countries, has the advantages that release of noxious substances in the exhaust gas can be remarkably reduced compared to the use of gasoline as a fuel. However, considering the current fuel supply system, use of a mixture of gasoline and alcohol as a mixed fuel would be more practicable than the use of alcohol alone as a fuel.
A vehicle equipped with an engine adaptable to such mixed fuel incorporates fuel blend ratio detection means in a fuel supply system of the vehicle for detecting a blend ratio of alcohol to gasoline, setting the ignition timing, air fuel ratio and the like according to the alcohol blend ratio supplied to the engine.
As the fuel blend ratio detection means, there are known a fuel blend ratio sensor, which detects the alcohol blend ratio of the mixed fuel by any method, and an O.sub.2 sensor, which detects the oxygen concentration of exhaust gas to indirectly estimate the alcohol blend ratio of the mixed fuel.
Even if the engine is adaptable to the mixed fuel, it is necessary to detect the alcohol blend ratio of the mixed fuel and correct the ignition timing and air fuel ratio according to the detection results, not so far as the alcohol blend ratio of the mixed fuel is constant.
Therefore, when a malfunction occurs in the detection by the fuel blend ratio detection means, the engine tends to be in poor operation condition or undergo a stalling and, in the worst case, may be damaged due to occurrence of frequent knocking.
To prevent such a problem, when a malfunction occurs in the detection by the fuel blend ratio detection means, the engine must be immediately stopped operating and repaired. However, it is impossible to perform this during running of the vehicle, and any other counter-measures are required.
U.S. Pat. No. 4,986,241 proposes a method in which, when a malfunction occurs in detection of fuel blend ratio by blend ratio detection means, the fuel blend ratio is temporarily fixed to a predetermined emergency blend ratio, and operation condition of the engine is then detected to correct the emergency blend ratio. However, in this method, since feedback control to correct the emergency blend ratio is complicated, a high processing power, expensive calculation apparatus is required.